Next generation physical layer security framework for wireless communications

Physical layer security became a very popular topic as new wireless technologies emerged and started to take over the major share of modern communication systems. The wireless communication networks use the unguided medium as the channel, which leads to the security vulnerability caused by the broadcast nature of the medium. Solving the problem of this vulnerability in the physical layer of wireless systems still remains a challenge. Current technologies use other solutions such as encryption to address the vulnerability of wireless physical layer. However, with the development of multilayer antennas, it became possible to address this issue in physical layer for the future systems. Thus, there have been many techniques proposed, which optimize the physical layer parameters to supplement the current security systems. However these methods are all stand-alone solutions, in other words they do not collaborate with other systems. This phenomenon makes them additional, and in most cases, optional. Hence, it is important to develop a new, unified physical layer security framework with increased awareness and adaptiveness. This way, implementing physical layer security can add real value to the system, especially when it is most needed. In this thesis, a new type of physical layer framework is proposed which combines the physical layer security techniques with multilayer awareness. The framework has the core modules, which calculate the system parameters using the inputs coming from enabled modules and transmits the signals. Each optional module add a new type of comprehension ability to the system. As the framework controls the transmission parameters, it is likely to be implemented within a wireless transmitter node. Therefore, it is named as the new-generation physical layer security framework. The new-generation framework has three major design features, namely flexibility, energy efficiency and adjustable security. The modular structure makes it tailorable so that the framework can support many applications and future technologies. The main purpose of the system is to optimize the transmission parameters to maintain successful legitimate communication while guaranteeing a predefined secrecy level, with the minimum power. The objective of minimizing power consumption is crucial for especially mobile devices, as their battery resources are limited. Lastly, the adjustable security provides a new level of efficiency to the system, where the system can differentiate between the input bits and provide better security for the important ones. The differentiation can be adjusted by the configuration of the modules and it is the main reason why the proposed method is more power efficient than the conventional physical layer systems. The following modules are described for the proposed next-generation framework. Core module, which consists of centralized computing module, beamforming and artificial noise modules, is responsible for the main functions of the system. This module itself acts like a basic physical layer security system, which is not aware of upper layer properties of the incoming stream. The encryption awareness module provides the detection of encryption method that is used in the system. The encryption methods change the physical layer properties of the system, hence the computing module adjusts the optimization accordingly and have a more accurate output. Also, it can change the encryption type or key length of the system to support the most power efficient solution. Application awareness module differentiates the incoming bits regarding their application-wise importance. This way, it becomes possible to differentiate the bits which are important such as a password in a HTTP GET or POST message. Also it can be used to assess and change the security requirement of the bits, for example it is possible to assign a higher security level for HTTP and lower level for HTTPS traffic. Channel estimation protection module provides information about the channel estimation error so that the core computing mode adjusts the thresholds to make sure the constraints will be met by investing more power. The module provides a higher reliability, which is very important for the real-life implementations. An arbitrary module can be added to the framework, which differentiates the bits. This may be required for integrating future protocols. The thesis provides an exemplary configuration sample for such an addition. The concept has been implemented in different scenarios. First, the spatiotemporal selective artificial noise scenario is considered. Here, the artificial noise has been designed as selective in the spatial (as in beamforming) and temporal (time selective) domains. The performance analysis showed that the spatioselective artificial noise can be more efficient than the existing isotropic artificial noise and space selective artificial noise scenarios. In the second scenario, channel estimation error protection was introduced for the artificial aided beamforming system. Threshold calculation was proposed in order to maintain the constraints with the minimal power, under the existence of channel estimation errors. The results show that the system can stay reliable with the assistance of a threshold, which comes with the prise of increased power consumption. In the third scenario, the encryption awareness was considered. In this scenario, the effects of using different encryption schemes, namely AES, DES and DESX are examined. Afterwards, the results for the framework with encryption awareness are given. Encryption changes the error properties of the system, e.g. one bit error on the encrypted stream cause multiple errors on the decrypted stream. If the system is encrypted, the number of errors between transmitter and receiver is not equal to the number of errors between encrypted and decrypted stream, which is referred to as the effective channel. Hence, the effective channel should be considered for accuracy. The results conclude that the encryption awareness brings enhanced power efficiency for the encrypted systems by preventing the power consumption caused by unnecessary use of artificial noise. In forth scenario, the application awareness is examined. The system has the application aware security feature, where only a specified part of video traffic is secured. The results show that the application awareness can selectively enhance security on important bits, and achieve higher efficiency by not wasting power on unimportant bits. The full duplex case is considered in the fifth scenario, where the legitimate nodes operate in full duplex mode and employ self interference cancellation. The fact that both nodes send information signals in full duplex communication, creates interference on the potential eavesdropper nodes. The results show that the same secrecy level can be achieved by the systems when the nodes operate in full duplex node. The effect of imperfect self interference cancellation is also shown. In the sixth scenario, the cognitive radio network application is employed. The most important physical layer attacks in cognitive radio networks are examined and beamforming based attack prevention is performed. The results show that the physical layer security methods can be employed as countermeasure to specific attacks, other than eavesdropping. In conclusion, this thesis proposes a physical layer security framework, which is called "next generation physical layer security framework" due to its modular and multilayer operating properties. The modules of the framework increase the power efficiency of the system, by bringing more intelligence for smarter handling of power invested to increase security. These modules are implemented on different scenarios and are shown to be successful in increasing efficiency. The framework can be implemented for different use cases with existing and also new modules, which can be added to the framework.